High Q, passive electronic filters are known in the art. One such filter is the one-half lattice resonator filter. A typical one-half lattice filter is illustrated in FIG. 1. The typical filter 100 comprises two resonators 101, 102, a transformer 104, and a compensation capacitor 106. The compensation capacitor 106 and a primary winding of the transformer 104 are connected in parallel between an input terminal 108 and a common terminal 110 of the filter 100. One resonator 101 is connected between the input terminal 108 and an output terminal 109, while the other resonator 102 is connected in series with a secondary winding of the transformer 104 between the output terminal 109 and the common terminal 110. In a typical configuration, each resonator 101, 102 comprises a quartz crystal having a corresponding resonant frequency (e.g., F.sub.1) and the transformer 104 comprises wires wrapped around a magnetic core, such as a toroid. The compensation capacitor 106 is typically a discrete capacitor selected to resonate with the inductance of the transformer's primary winding.
In a common application, the filter 100 is used as a bandpass filter centered at an intermediate frequency of a radio receiver. To provide a bandpass response, the resonant frequencies of the crystals 101, 102 reside in the passband of the filter 100 and are typically selected such that F.sub.1 &gt;F.sub.2. When an input signal is applied to the input terminal 108, the filter 100 passes frequency components of the input signal that reside in the filter's passband and attenuates frequency components that reside in the stopband (i.e., outside the passband). The attenuation of the unwanted frequency components is provided by a 180 degree phase shift introduced by the transformer 104 (as denoted in the FIG. by the 1:-1 notation above the transformer 104). The phase shift provides essentially equal, but opposite, voltages of stopband frequency components in the branches that include the resonators 101, 102. Thus, when these voltages are combined at the output terminal 109, the net result is effectively zero transmission in the stopband. As is known, the actual filter rolloff (i.e., the slope of the attenuation outside the passband) is determined by filter topology (e.g., Butterworth, Chebyschev, etc.) and the number of crystals that constitute the resonators 101, 102.
By using a crystal filter as discussed above, high Q bandpass filters having center frequencies up to about 100 MHz are readily realizable. However, at frequencies above 100 MHz, the parasitic capacitances of the transformer 104 prevent the transformer 104 from providing the desired 180 degree phase shift, thereby degrading the attenuation capability of the filter. In addition, the inherently large size of the transformer 104 prohibits it from being constructed within a monolithic microwave integrated circuit (MMIC). MMICs are commonly used in current microwave and sub-microwave radio receivers to reduce receiver size and improve reliability.
Therefore, a need exists for a high Q filter that is readily realized at frequencies above 100 MHz and that is compatible with integrated circuit technologies.